For the verizon iphone 4, mines was doing the same for about a month before i got tired of it being broken and came on here to look for an answer, since there was no post on how to fix or locate the issue on the iphone 4 cdma version, i just looked around for any exposed metal that looked like there should be some sort of resistor or capacitor on and took to testing, finally found the culprit after many hard resets and soft resets and wiping the phone disassembly and re-assembly, anyways:
for iPhone 4 CDMA version, the one i found that prevented the camera from working (shutter stuck) is a small resistor on the mobo, location of it is at the end of the camera's connection ribbon. If you lift the ribbon connector up you will see a straight row of tiny 3 capacitors, I didnt even fixed mines permanantly yet, but the way i fixed it work so im keeping it that way until it stops then ill go and solder it in.
I just cut one strand of speaker wire copper, about 1 milimeter, and folded that in half, set it where the missing capacitor is and put a peice of tape on it (very small strip) then rolled up a small peice of cotton to provide pressure once you snap the camera ribbon connector on top, it will firmly hold it in place.
Hey its ghetto, but I've gone a long time without it, and now its working. all from things i found around my office.